The underwater granulation of thermoplastic materials is conducted by cutting strands of the plastic material exiting from the openings of bores in the die plate with cutting knives acting on the cutting surface of the die plate under compressive pressure. Metal materials containing carbide particles of a size of less than 10.mu. and a volume of more then 50% are known for forming the cutting surface of the die plate. However, these hard material alloys have a low coefficient of heat expansion which is a disadvantage when compared with iron-based alloys, for example, with a martensitic structure. The stresses that arise between the interfaces of the materials lead to formation of stress cracks in the materials and thus to formation of plastic granulate of lower quality and to premature breakdown of the materials.
In U.S. Pat. No. 3,271,822, shaped pieces of sintered tungsten carbide are introduced into a recess in the die plate. These pieces are separated from each other and from the die plate by thin metal strips of silver and copper and are securely soldered under vacuum, so that the gaps between the shaped pieces are filled.
This insertion of individual shaped pieces into the die plate is not permanent due to the wear of the gaps filled with the silver solder whereupon these then act as expansion gaps, as washouts of the expansion gap occur after a certain amount of time. This has a considerable adverse effect on the cutting process of the plastic strands. It leads to nonuniform and "burned" granulate at the output or cutting surface of the cutting plate.
In another die plate disclosed in U.S. Pat. No. 3,599,286, the cutting surface is provided with several ceramic layers and thin metal strips are placed between and combined with the ceramic layers by flame spraying, these strips sealing the die plate openings and forming a heat insulation in a sandwich-type structure. The upper ceramic layer forming the cutting surface of the cutting plate is in danger of breaking due to its brittleness, since heat stresses, caused by the cooling effect of the granulating water or by the input of heat resulting above the dies, are not intercepted. This also leads to expansion cracks in the cutting surface.